BUG/MINOR: listener: null pointer dereference suspected by coverity

BUG/MINOR: listener: null pointer dereference suspected by coverity

Please refer to GH #1859 for more info.
Coverity suspected improper proxy pointer handling.
Without the fix it is considered safe for the moment, but it might not
be the case in the future as we want to keep the ability to have
isolated listeners.

Making sure stop_listener(), pause_listener(), resume_listener()
and listener_release() functions make proper use
of px pointer in that context.

No need for backport except if multi-connection protocols (ie:FTP)
were to be backported as well.